The present invention relates to thermophotovoltaics and more particularly to the use of new low bandgap Ga.sub.1-x In.sub.x Sb photovoltaic cells sensitive to IR radiation down to 2.0 .mu.m. These new cells allow the use of lower temperature emitters. (FIG. 7 from Ref. 1 shows the effects of emitter temperature on the wave length of the emitted radiation.) Previous silicon cells required the operation of IR emitters at much higher temperatures where materials problems have severely limited the emitter lifetime. Further, to obtain emitter temperatures in the 1300 to 1800 K range requires even higher flame temperatures, and flame temperatures above about 1300 K are extremely difficult to achieve without the use of oxygen (which is unacceptably expensive) or preheating of the combustion air, usually by regeneration. (See Ref. 2, Chapters 5 and 16.) Thus to obtain a significant fraction of the combustion energy in usable IR radiation, the unit must make use of regenerative heating of the combustion air together with low bandgap cells. These new low bandgap cells may be wired in series strings and combined with a novel burner/emitter designed to give a nearly uniform temperature along its length in order to maximize cell string current and power output. The cell strings and burner/emitter are integrated with IR reflectors to form a compact, quiet, light-weight DC electric power supply which can be used for off-grid electric power for recreational vehicles, mountain cabins, or third world village homes. In colder climates, the unit can be used for heating as well as electric power.
One version of the unit is cylindrical in shape with a diameter of 5 to 10 in. and a height of 18 to 36 in. See FIGS. 2 to 6. Depending on the number of cell strings provided with this unit, it is designed to generate from 300 W to 1 kW.
Various attempts to fabricate practical and economically interesting thermophotovoltaic (TPV) power generators have been reported over the years. In 1986, D. G. Pelka, A. Santos, and W. W. Yuen described both earlier efforts and their attempt to design and fabricate a natural-gas-fired thermophotovoltaic system (Ref. 3). Their desk-top-sized unit used a rotating drum with a porous particle bed serving as the high temperature emitter operating at &gt;2000 K. Silicon TPV cells were mounted inside the rotating drum. They noted that for silicon cells and an emitter temperature &lt;2600 K, over 76% of the black body radiation is in unusably long wavelengths ( &gt;1.1 .mu.m) energy. Their design was complicated by their attempt to design a very high temperature emitter as required by the silicon TPV cells. Without very high temperature emitters, TPV systems based on silicon cells are both inefficient and operate at low power densities. Selective emitters based on rare earth oxides have been described (U.S. Pat. No. 4,976,606 Ref. 4) which improve efficiencies but still suffer from low power densities at practical emitter temperatures.
In 1989, L. M. Fraas et. al. described a new GaSb photovoltaic cell sensitive in the IR out to 1.8 .mu.m. This cell was designed to be used with concentrated sunlight as an infrared booster cell in tandem with GaAs solar cells (U.S. Pat. No. 5,096,505, Ref. 5). In 1989, M. D. Morgan, W. E. Horne, and A. C. Day (NASA SPRAT conference, Ref. 6) proposed using GaSb cells in combination with a radioisotope thermal generator for space electric power, and in 1991 O. L. Doellner proposed using GaSb cells looking at jet engine plumes to replace alternators on jet aircraft (Ref. 7). As of this writing, neither Morgan nor Doellner has built a TPV generator using GaSb cells.
It now seems timely to take a fresh approach to the design of a compact gas-fired TPV generator. It is clear that the longer wavelength response of the GaSb cell will allow the use of lower temperature emitters. However, several problems must still be solved. First, when photovoltaic cells are wired together in series in order to generate a required voltage, e.g. 12 V, it is important that they each receive the same amount of IR radiation. Otherwise, the cell string current is limited by the cell with the lowest IR generated current. This translates to a requirement to tailor the temperature profile over the length of the emitter. A uniform temperature would give a first approximation, but end effects will probably require special handling of both the shape of the end reflectors and the burner jet pattern to obtain the desired uniform radiation energy input along the length of each cell string. Second, the energy conversion efficiency is not only controlled by the TPV cell bandgap and response to the IR; exhaust gas heat losses up the stack will be very large without provision for regenerative heating of the combustion supply air by the exhaust gas. This will also increase the flame temperature and permit higher emitter temperatures, which will in turn increase the cell output per unit of cell area, and thus reduce the size, weight, and cost. Third, the low bandgap TPV cell temperature must be maintained near room temperature in order to preserve high cell conversion efficiency. Fourth, the IR energy from the emitter has to be efficiently coupled to the TPV cell strings. Fifth, it may be desirable to tune the low bandgap response even somewhat further into the infrared.